The LCD (Liquid Crystal Display) possesses many advantages of being ultra thin, power saved and radiation free. It has been widely utilized in, such as mobile phones, Personal Digital Assistant (PDAs), digital cameras, laptop screens or notebook screens.
Most of LCDs on the market are back light type LCDs, comprising a Back light module and a liquid crystal panel combined with the Back light module. The working principle of the liquid crystal panel is to locate liquid crystal molecules between two parallel glass substrates, and a plurality of vertical and horizontal tiny electrical wires are between the two glass substrates. The light of back light module is reflected to generate images by applying driving voltages to control whether the liquid crystal molecules to be changed directions. The liquid crystal panel itself does not emit light and needs the back light module to provide light source for normally showing images. The back light modules can be categorized as two types, a side light type back light module and a direct light type back light module according to the difference of the light source incident positions. The side light type back light module is to locate the back light source LED light bar at the back plate edge behind the liquid crystal panel, and the light emitted by the LED light bar enters the LGP (Light Guide Plate) from the incident surface at one side of the light guide plate. After reflection and diffusion, the light goes out from the illuminating surface, and then, a surface light source is formed by the optical film set to provide the same to the liquid crystal panel.
The brightness uniformity of the back light module has great influence to the display uniformity of the liquid crystal module. The brightness uniformity of the back light module is generally defined by a ratio of the brightness minimum and the brightness maximum of 9 points, 13 points or 25 points. When the ratio is larger than some value (such as 80%), the brightness distribution of the back light module reaches the standard or reaches the basic uniformity. Then, the liquid crystal module is assembled with combining the liquid crystal panel. Such design forgets one pint, which the transmittance distribution of the liquid crystal panel is not entirely uniform. As shown in FIG. 1, as the brightness distribution of the back light module 100 is uniform and after combining the liquid crystal panel 200 to assemble the liquid crystal module 300, the brightness of the liquid crystal module 300 is high where the transmittance of the liquid crystal panel 200 is high, and the brightness of the liquid crystal module 300 is low where the transmittance of the liquid crystal panel 200 is low. Therefore, even the back light module 100 is designed to be very uniform, the brightness of the liquid crystal module 300 after combining the liquid crystal panel 200 still has possibility to be nonuniform.